A. Field of the Invention
This invention relates to gas separation membranes.
B. Prior Art
The separating, including upgrading of the concentration of, at least one gas from a gaseous mixture is an especially important procedure in view of the demands on the supplies of chemical feedstocks. Frequently these demands are met by separating one or more desired gases from gaseous mixtures and utilizing the gaseous products for processing. Applications have been made employing separation membranes for selectively separating one or more gases from gaseous mixtures. To achieve selective separation the membrane exhibits less resistance to transport of one or more gases than of at least one other gas of the mixture. Thus, selective separation can provide preferential depletion or concentration of one or more desired gases in the mixture with respect to at least one other gas and therefore provide a product having a different proportion of the one or more desired gases to the at least one other gas than the proportion in the mixture.
In order for selective separation of the one or more desired gases by the use of separation membranes to be commercially attractive, the membranes must satisfy several criteria so that the use of the separation procedure is economically attractive. For instance the membranes must be capable of withstanding the conditions to which they may be subjected during the separation operation. The membranes also must provide an adequately selective separation of one or more desired gases at a sufficiently high flux, that is, permeation rate of the permeate gas per unit surface area. Thus, separation membranes which exhibit adequately high selective separation but undesirably low fluxes, may require such large separating membrane surface area that the use of these membranes is not economically feasible. Similarly separation membranes which exhibit a high flux but low selective separation are also commercially unattractive.
Membranes which are not capable of maintaining the desired high flux and selective separation over extended periods of time in adverse operating environments are likewise economically not feasible. Adverse environmental conditions include extremes of temperature, pressure and concentrations of chemical contaminants. Accordingly, work has continued to develop gas separation membranes which can provide both an adequately selective separation of one or more desired gases at a sufficiently high flux for an extended period of time under adverse environmental conditions such that the use of these gas separation membranes is economically feasible.
In general, the passage of a gas through a membrane may proceed through pores, i.e., continuous channels for fluid flow in communication with both feed and exit surfaces of the membrane (which pores may or may not be suitable for separation by Knudsen flow or diffusion); in another mechanism, in accordance with current views of membrane theory the passage of a gas through the membrane may be by interaction of the gas with the material of the membrane. In this latter postulated mechanism, the permeability of a gas through a membrane is believed to involve the solubility of the gas in the membrane material and the diffusion of the gas through the membrane. The permeability constant for a single gas is presently viewed as being the product of the solubility and diffusivity of that gas in the membrane. A given membrane material has a particular permeability constant for passage of a given gas by the interaction of the gas with the material of the membrane. The rate of permeation of the gas, i.e., flux, through the membrane is related to the permeability constant, but is also influenced by variables such as the membrane thickness, the physical nature of the membrane, the partial pressure differential of the permeate gas across the membrane, the temperature of the gas and the like.
It has been generally observed that materials which exhibit good separation of gases often have lower permeability constants compared to those of materials exhibiting poor separation of gases. In general, efforts have been directed to providing the material of a gas separation membrane in as thin a form as possible, in view of the low permeabilities, in order to provide adequate flux yet provide a membrane as pore-free as possible, such that gases are passed through the membrane by interaction with the material of the membrane.
One approach to developing separation membranes suitable for gaseous systems has been to provide composite membranes having a superimposed membrane supported on an anisotropic porous support wherein the superimposed membrane provides the desired separation, i.e., the superimposed membrane is semi-permeable. The superimposed membranes are advantageously sufficiently thin, i.e., ultrathin, to provide reasonable fluxes. The essential function of the porous support is to support and protect the superimposed membrane without harming the delicate, thin superimposed membrane. Suitable supports are desirably porous to provide low resistance to permeate passage after the superimposed membrane has performed its function of selectively separating the permeate from the feed mixture. Klass et.al., U.S. Pat. No. 3,616,607; Stancell et.al., U.S. Pat. No. 3,657,113; Yasuda, U.S. Pat. No. 3,775,303; and Browall, U.S. Pat. No. 3,980,456 exemplify gas separation membranes having superimposed membranes on a porous support.
Such composite membranes for gas separations have not been without problems. For instance, Browall discloses that in the fabrication of composite membranes of ultrathin films fine particles (i.e., particles below about 3000 Angstroms in size) may be deposited under or between preformed ultrathin membrane layers and, because of their large size in comparison to the ultrathin membranes, puncture the ultrathin membranes. Such breaches reduce the effectiveness of the membrane. The Browall patent discloses applying a preformed organopolysiloxane-polycarbonate copolymer sealing material over the ultrathin membrane to cover the breaches caused by the fine particles. Browall also discloses employing a preformed layer of the organopolysiloxane-polycarbonate copolymer between the ultrathin membrane and the porous polycarbonate support as an adhesive. Thus the composite membranes of Browall are complex in materials and techniques of construction.
A major improvement in gas separation membranes is disclosed by Henis et.al. in U.S. Pat. No. 4,230,463 which pertains to particular multicomponent membranes for gas separations comprising a coating in contact with a porous separation membrane wherein the separation properties of the multicomponent membranes are principally determined by the porous separation membrane as opposed to the material of the coating. Such multicomponent membranes for the separation of at least one gas from a gaseous mixture can exhibit a desirable selectivity and still exhibit a useful flux. Moreover, such multicomponent membranes for gas separation can be fabricated from a wide variety of gas separation membrane materials allowing great latitude in selecting such a membrane material which is advantageous for a given gas separation. The desired combinations of flux and selectivity of separation can be provided by the configuration and methods of preparation and combination of the components. However, this membrane suffers the disadvantage that it is easily damaged in the presence of certain contaminant gases frequently found in gas streams.
Despite such advances in gas separation membranes it would be advantageous to construct any membrane, including such multicomponent membranes, from materials that have normally high flux and the desired selectivity of separation but also have stable environmental resistance. That is, the membranes should have high structural strength, toughness, abrasion and chemical resistance to remain functional under extremes of pressure and differential pressure. In this regard such membranes are often required to operate with very high pressure differentials across the membrane.
Depending on the operating environment chemical resistance is also desirable for instance resistance to hydrocarbons, ammonia, water and acid gases such as carbon dioxide and hydrogen sulfide is often required. Such chemicals may have a tendency to dissolve or plasticize the material of gas separation membranes resulting in deterioration of the membrane or densification of an otherwise asymmetric structure.
Polyarylene oxides have been recognized as material of some potential in the membrane separation field. For instance, Robb in U.S. Pat. No. 3,350,844 disclosed that polyarylene oxide membranes, for instance membranes of 2,6-dimethylphenylene oxide membranes, have unique properties such as a high separation factor and flux together with strength and ability to form thin films. Robb further discloses that factors such as temperature, pressure, elongation of oriented membrane material, the amount of crystallinity, among others, in the polyarylene oxide resin, may effect permeability. In this regard polyphenylene oxide resins have a low resistance to most common organic solvents. Aromatic and chlorinated hydrocarbon solvents dissolve polyphenylene oxide polymers, while other solvents and solvent vapors induce crazing in molded polyphenylene oxide parts under stress thus causing almost complete loss of strength.
See also Kimura, U.S. Pat. Nos. 3,709,774; 3,762,136; and 3,852,388 which relate to membranes of polyxylene oxide with the same apparent disadvantages.
An alternative form of polyarylene oxide membranes is disclosed by Salemme in U.S. Pat. No. 3,735,559 where various ionic forms of a sulfonated polyxylylene membrane are disclosed. Among the disadvantages discussed are that it is necessary to preshrink such membranes to avoid rupturing; the hydrogen ion form is unstable and may undergo sulfone formation resulting in cross-linking or may, in the presence of water, undergo hydrolysis with the liberation of sulfuric acid; various counter ion salt forms of the membrane are stable and will avoid detrimental cross-linking but such membranes may densify in the presence of water.
In summary coated suitable polyarylene oxide membranes have not been provided for gas separations which can exhibit sufficient flux and selectivity of separation for general commercial operations in the presence of adverse environmental conditions as the presence of chemical contaminants, extremes of differential pressure and temperature.